The present invention relates to a reinforced cement sheet product which does not contain asbestos but which contains other fibers and the combination of clay and thickener so as to permit satisfactory fabrication thereof on a machine in which a slurry is formed on a screen and subsequently vacuum-filtered through a screen and/or felt.
By way of background, in the past asbestos-cement compositions have been formed into products such as siding shingles, flat sheets, corrugated sheet products, and the like. These sheets generally contained Type 1 Portland cement binder in the amount of about 55%, silica and/or filler in the amount of about 30%, and asbestos fibers in the amount of about 15%. The products had many desirable qualities, including high strength, exterior durability, easy application, and the ability to be colored in various colors. In addition, the existence of asbestos provided formation of a suitable "filter mat" which resulted in (1) slowing down the drainage rate and (2) a low percent of solids loss of the various fines during manufacture. The high drainage time permitted the product to be formed properly during fabrication, and the low solids loss resulted in a stronger product because relatively little of the cement fines were lost. In addition, asbestos has always displayed the unique characteristic of good dispersion in a cementitious/water slurry because of its hydrophilic nature. This means that the asbestos fibers intimately mixed with the cement particles and remained in suspension without settling out or floating to the top of the slurry. However, in recent years governmental regulations have restricted the use of asbestos due to its carcinogenic effects. Therefore, attempts have been made to find substitutes for asbestos. However, various other fibers were not capable, without further modification in formulation, of forming suitable filter mat from the slurry, and providing a relatively high drainage time and a relatively low solids loss during fabrication. One such attempt is disclosed in U.S. Pat. No. 4,040,851 which utilizes cotton fibers instead of asbestos fibers, along with Portland cement, silica, and hydroxyethyl cellulose thickener. However, a formula of this type has not been found to have a desirable combination of sufficiently high drainage time and a sufficiently low solids loss. It has also been found that various synthetic fibers, such as polyethylene, polypropylene, etc. and some natural fibers cannot be used instead of asbestos because they will not disperse properly in the cement-silica water slurry. In this respect, they tend to float to the top of the slurry because they have a lower specific gravity than water, and therefore they will not form a homogeneous mat on the cylinder of a wet forming machine. It is with overcoming the foregoing deficiencies of the prior art that the present invention is concerned.